[Overbyte]

I'm sure there must be a thread on this somewhere but my question is this:

Spell defense is used against hostile spells cast at you or at targets that are within your line of sight (using the same rules as for targeting spells) that you decide to protect with spell defense. Declaring this protection is either a Free Action or, if you don’t have one left, an Interrupt Action that reduces your Initiative Score by 5. Each Combat Turn, you have a pool of dice for spell defense equal to your  Counterspelling skill. When a spell is cast, you may choose to use some or all of your Counterspelling dice to defend against an incoming
spell. SR5 - p.295

Not to get into the same conversation as the other thread, but how do you even know to use your Spell Defense? Do you have to see the mage casting or be astrally perceiving to see the spell coming at you and/or your buddies?

Mostly I just allow players to do it when they alternative is something really bad happening to them, but I'm wondering how others handle this.

[Beta]

I had the same question.  After trying a couple of different things, what I settled on as my house rule is roughly

"Spells are delicate things, and should be easy enough to mess with ... except that they are both very hard to sense, and happen in the blink of an eye.  A mage with counterspelling has learned to pay attention to the mana ripples that preceed a spell being cast nearby, and to pull on the mana themselves in hopes of throwing off the one doing the casting."

"If the counter-spelling mage is
-astrally perceiving,
-can buy two hits, and
-has line of sight on the caster, or line of sight on the target item/location of anything other than an indirect combat spell
then they know the class of spell (combat, illusion, etc).  If they don't meet one of those criteria they only know that a spell is being cast nearby.  They can choose to counter it either way."

Mind you, I also house ruled that counterspelling successes are deducted from caster successes (so no choosing who you protect and who you don't).  This makes counterspelling effective against indirect AOE combat spells (notably resulting in more scattered spells, which to me is fun), cuts down on the total dice to roll, and can even have the result of a spell fizzling completely.